Many drive systems, such as vehicle liftgate openers, include a clutch which operates to disengage the electric drive from the mechanism which normally operates the liftgate so that manual operation of the liftgate can be achieved. Conventionally, such clutches have been mechanical toothed clutches which provide the necessary torque handling capabilities required for drive systems for operating heavy devices such as vehicle liftgate openers.
While such toothed clutches have been widely used, they do suffer from disadvantages. In particular, such clutches must only be engaged and disengaged in no-load conditions, with no relative movement between the tooth rings, or damage to the clutch components can occur. Further, in the event of an overload condition occurring in a drive system including a toothed clutch, there is a high probability that damage will occur to components in the drive system as the clutch cannot slip and thus the drive components must transfer the overload forces. Also, toothed clutches are relatively expensive to manufacture.
A known alternative to mechanical toothed clutches is to employ magnetic friction clutches wherein a driving disc is brought into frictional contact with a driven disc by an electromagnetic field. When the electromagnetic field is energized, a normal force is created through the discs urging the driven and driving discs together and torque at the driving disc is transferred to the driven disc by the frictional force between the discs. Conversely, when the electromagnetic field is de-energized, the normal force is removed and substantially no torque is transferred between the driven and driving discs.
Unlike toothed clutches, when magnetic friction clutches are subjected to overload conditions and/or dynamic loads, they can momentarily slip, preventing mechanical damage to the clutch and/or driving and driven devices.
While prior art magnetic friction clutches are used in a variety of applications and offer advantages over toothed clutches, they also suffer from disadvantages when the space into which the clutch is to be installed is limited and/or the torque that the clutch must transfer is relatively high. Generally, the torque transfer capability of known magnetic friction clutches is related to the diameter of the driven and driving discs and higher torque transfer capabilities can only be achieved with relatively large disc diameters. In particular, known magnetic friction clutches have not been suitable for vehicular applications, such as liftgate openers, which require a relatively high torque transfer capability and yet have limited volume in which the clutch can be installed.